A nitride semiconductor such as gallium nitride (GaN), aluminum nitride (AlN), and indium nitride (InN), or a material composed of a mixed crystal of GaN, AlN and InN generally has a wide band gap. These materials are utilized as a high-power electronic device, a short-wavelength light emitting device or the like. Among these, a technology associated with a field-effect transistor (FET), specifically, with a high electron mobility transistor (HEMT) has been developed as a high-power device (e.g., Patent Document 1). A high electron mobility transistor (HEMT) including a nitride semiconductor may be utilized for a high-power and high-efficiency amplifier, a high-power switching device, and the like.
The HEMT having a nitride semiconductor generally includes an aluminum gallium nitride/gallium nitride (AlGaN/GaN) heterostructure formed on a substrate, in which a GaN layer serves as an electron transit layer. Note that the substrate may be formed of sapphire, silicon carbide (SiC), gallium nitride (GaN), silicon (Si), and the like.
The GaN, which is a kind of the nitride semiconductor, includes a high saturation electron velocity or a wide band gap. Hence, the GaN may be able to acquire superior pressure resistance and exhibit excellent electric characteristics. Since the crystal structure of the GaN is a hexagonal wurtzite structure, the GaN is polarized in a [0 0 0 1] direction parallel to a c-axis (wurtzite form). Further, when the AlGaN/GaN heterostructure is formed, piezoelectric polarization may be induced by lattice strain between the AlGaN and GaN. As a result, a highly-concentrated two-dimensional electron gas (2DEG) may be generated near an interface of the GaN layer serving as a channel. Hence, the high electron mobility transistor (HEMT) utilizing the GaN may be developed as a potential high-power device.
However, when the high electron mobility transistor (HEMT) utilizing the GaN is in an off status that is less than or equal to the pinch-off voltage, a current may pass through a lower part of the electron transit layer directly beneath the gate electrode so that the current may be leaked from the drain side to the source side. Specifically, as illustrated in FIG. 1, the HEMT having the GaN includes a buffer layer 912, an electron transit layer 913, and an electron donation layer 914 formed on a substrate 911. The HEMT further includes a gate electrode 921, a source electrode 922 and a drain electrode 923 on the electron donation layer 914. Note that the electron transit layer 913 is formed of GaN, the electron donation layer is formed of AlGaN. Hence, 2DEG 913a is formed near the interface between the electron transit layer 913 and the electron donation layer 914.
In general, the electron transit layer 913 is formed such that the electron transit layer 913 is sufficiently thick for securing crystallinity. However, when the electron transit layer 913 is thick, the electric field generated by the voltage applied to the gate electrode 921 may not reach a part or an area directly beneath the gate electrode 921, which may facilitate the generation of the leakage current in the lower part of the electron transit layer 913. That is, a depletion region 919 formed by the application of the gate voltage to the gate electrode 921 or the like may not reach the lower part of the electron transit layer 913, which may facilitate the generation of the leakage current in a direction indicated by an arrow in the lower part of the electron transit layer 913 illustrated in FIG. 1. When such a leakage current is increased in the HEMT having GaN that is utilized as a high-power amplifier, the amplification efficiency may be lowered.
Further, the HEMT having GaN is generally susceptible to being in a normally-on status due to the highly-concentrated two-dimensional electron gas (2DEG). Hence, the HEMT having GaN may not easily acquire a normally-off characteristic. Numerous semiconductor devices in the current power electronics market have a normally-off characteristic. Hence, it is highly preferable that the HEMT having GaN have a normally-off characteristic in view of compatibility between the HEMT having GaN and the semiconductor devices.
Accordingly, there are disclosed various methods for controlling the leakage current. For example, the leakage current may be controlled by thinning the electron transit layer 913 formed of GaN, or by doping impurity serving as an acceptor such as Mg or Fe to the lower part of the electron transit layer 913 so as to increase the resistance of the lower part of the electron transit layer 913 (e.g., Patent Document 1). Further, there is disclosed various methods for maintaining the normally-off status. For example, the normally-off status may be maintained by forming a Mg-doped low resistance p-type GaN layer between an electron donation layer and a gate electrode such that the generation of 2DEG directly beneath the gate electrode may be suppressed by holes supplied from the low resistance p-type GaN layer (e.g., Patent Document 2).